1. Field of the Invention
The present invention relates to an information reproducing apparatus and an information reproducing method for reproducing information from a large-capacity optical information recording medium having a high super-resolution effect.
2. Background Art
With the recent development of the information society involving optical communications, the construction of a communication system that enables high-speed communication of large-capacity information is being demanded. As an optical device indispensable for achieving such large-capacity, high-speed optical communication, an optical information recording/reproducing apparatus for storing large-capacity optical information is used. Further, in association with higher resolution, such as digitalization or higher definition of an image for television, it is urgent to develop a large-capacity optical information recording/reproducing apparatus capable of recording such image in a high-resolution state for long periods of time.
Currently, as an optical information recording medium, DVDs having a capacity of 4.7 GB per side are widely used for handling large-capacity moving images such as videos, besides computer applications. With regard to such DVD, in addition to a read-only ROM (DVD-ROM), on the substrate of which information is directly written, a rewritable recording/reproducing medium is being put to practical use. The development for improving the recording density of such optical information recording medium is being carried out, and as a means for achieving higher density of information recording, a laser light with a wavelength of 650 nm that is shorter than that (780 nm) of a laser used for CDs or the like is used. However, in order to deal with large-capacity information, such as computer graphics or digital high-definition images, it is necessary to achieve a recording density four or five times higher than such medium. For this, an optical disk utilizing a blue semiconductor laser with an even shorter wavelength (405 nm) is being developed, and an optical disk having a capacity of 23.3 GB per side is being put to practical use.
As a technology for achieving even higher capacity for optical disks, a multilayer recording system, a multi-value recording system, a super-resolution recording system, and the like are being developed. Among such next-generation technologies for achieving a higher capacity, the super-resolution recording system is one of the most effective technologies for achieving a higher capacity.
The super-resolution recording system is a recording system in which a recording surface is irradiated with a laser beam, the diameter of the laser beam emitted being reduced by a laser beam condensing function or a mask function due to a super-resolution layer, and it is a large-capacity recording system utilizing reversible change in optical constants (refractive index (n) and extinction coefficient (k)) of a super-resolution layer formed in a multilayer film structure including a recording layer, a protective layer, a reflective layer, and the like in an optical disk. When the super-resolution layer is irradiated with a laser light used for reading and writing, the temperature is increased or the layer is caused to be excited due to photon absorption by the light. The refractive index and extinction coefficient are reversibly changed only during the laser light irradiation, and the original state is restored when the laser light irradiation ceases. In an optical disk, information is reproduced by irradiating the disk with a laser light, and determining a recorded part and a non-recorded part by the amount of light that returns to a pick-up after reflected by the disk. By utilizing such change in reversible optical constants of the super-resolution layer, the region of the light that returns to the pick-up can be made smaller than the area irradiated with a general irradiation laser light. Namely, by reducing the region to be read based on an optical masking effect, it is possible to improve resolution. Meanwhile, the extinction coefficient (k) used herein is the amount proportional to optical absorption coefficient of a material, and the larger the absorption coefficient of a material, the greater the value will be. Further, the two constants, that is, the refractive index (n) and extinction coefficient (k), are collectively referred to as optical constants.
As disclosed in JP Patent Publication (Kokai) No. 10-340482 (1998) A, a cobalt oxide-based thin-film material or the like has been used for such super-resolution layer. A large-capacity optical disk can be obtained through a super-resolution effect based on a large change in refractive index of the thin film. In a current optical information recording/reproducing apparatus, when optical information is reproduced, continuous light (CW light: Continuous Wave light) or reproduction laser light that is superimposed at a high frequency of approximately 400 MHz is emitted. Thus, if the complex refractive index of the above super-resolution layer is changed, and the power of the reproduction light is increased until the super-resolution effect is obtained, heat is accumulated on the optical information recording medium due to the laser light irradiation, thereby causing a problem that a broad heat distribution develops in the laser beam spot and a super-resolution mask with high contrast cannot be formed. Furthermore, there is a concern that a thin film or a recording pit on the medium deteriorates due to accumulation of the heat, whereby operation characteristics of repetitive reproduction deteriorate.
JP Patent Publication (Kokai) No. 10-40547 (1998) A or the like discloses a reproduction system utilizing a pulsed light, as a system for improving the super-resolution effect by preventing the accumulation of such heat in a medium and making the temperature gradient in a beam spot to have a sharp inclination.
Patent Document 1: JP Patent Publication (Kokai) No. 10-340482 (1998) A
Patent Document 2: JP Patent Publication (Kokai) No. 10-40547 (1998) A